The present invention relates to an electrophotographic printer, to a print head that illuminates a photosensitive drum in the electrophotographic printer, and to an array of light-emitting diodes used as light sources in the print head; more particularly to a printer, print head, and array with a resolution of one thousand two hundred dots per inch (1200 DPI).
The background art will be briefly described with reference to FIGS. 19 to 21. FIG. 19 shows an oblique view of a typical array of light-emitting diodes or LEDs 1. The LEDs 1 are formed in an n-type compound semiconductor substrate 2 covered by a transparent insulating film 3. Each LED 1 has an electrode 4 leading to a window 6 in the insulating film 3. A pn junction is created by diffusion of a p-type impurity through the window 6. Referring to FIG. 20, diffusion proceeds laterally as well as vertically, so the surface extent 7 of the p-type diffusion region extends beyond the edges of the window 6. Light is emitted from the entire surface extent 7 of the p-type diffusion region, and passes substantially unobstructed through the insulating film 3, but part of the surface extent 7a is covered by the electrode 4, which is opaque.
The electrode 4 makes contact with the surface of the p-type diffusion region in a contact region 6a inside the window 6. The contact can be seen more clearly in FIG. 21, which is a sectional view through line C-C' in FIG. 20. FIG. 21 also shows the n-type substrate 2, the p-type (diffusion region 8, a common electrode 9 disposed on the underside of the substrate 2, and the pn junction 5 between the p-type and n-type regions.
LED arrays of the type shown in FIGS. 19 to 21 are used as light sources in the print, heads of electrophotographic printers. In a conventional printer with a resolution of, say, four hundred dots per inch (400 DPI), the array pitch is sixty-three micrometers (63 .mu.m). Each window 6 is about thirty micrometers wide and about forty micrometers long (30 .mu.m.times.40 .mu.m) . The surface extent 7 of the p-type diffusion region 8 is about forty micrometers wide and about fifty micrometers long (40 .mu.m.times.50 .mu.m). The contact region 6a must be large enough so that the electrical resistance between the electrode 4 and the p-type diffusion region 8 is not too high. The contact region 6a has a surface area of, for example, one hundred fifty square micrometers (150 .mu.m.sup.2). The covered part 7a accounts for about one-tenth of the total surface extent 7 of the diffusion region; that is, the covering ratio is about 0.1.
In a high-resolution printer, with a resolution of 1200 DPI or greater, the above dimensions must be reduced. If they are reduced proportionally, the area of the contact region 6a becomes too small, the contact resistance becomes too high for adequate current to flow, and insufficient light is emitted. If the same contact area is maintained when the other dimensions are reduced, however, then the covering ratio becomes so high that light emission is again insufficient.
The problem of creating a satisfactory LED array for a high-resolution printer is not simply a matter of determining how to scale down the covering ratio. Various other factors have to be considered, including the spacing between adjacent light-emitting regions, the spacing between the light-emitting regions at the ends of the array and the physical edges of the substrate, and the effect of these spacings on the quality of the printed output. The prior art has lacked a satisfactory solution that takes these factors into account.